A scientist using a powerful NASA space telescope says he may have finally detected dark matter.
Dark matter is thought to make up about 27% of all the matter in the Universe, and it acts as a sort of gravitational 'glue' holding whole galaxies together.
More on dark matter
But no-one has ever seen or directly detected it.
Dark matter remains a mystery, but Professor Tomonori Totani from University of Tokyo believes he may have finally detected it.
The origins of dark matter
To understand where the theory of dark matter comes from, we need to go all the way back to the 1930s, when astronomer Fritz Zwicky observed that galaxies were rotating faster than their overall mass should allow.
He calculated that, if galaxies were made up of just the matter that we can directly observe – stars, gas, dust etc. – there wouldn't be enough gravity to prevent the galaxies ripping apart due to their rapid rotation.
There must, Zwicky reasoned, be some other substance in the galaxies holding them together, but which we can't see or directly detect. This became known as dark matter.
Over the 100 years since the first theories of dark matter, scientists have been able to indirectly observe dark matter through its effect on observable matter.
But it can't be directly observed because it doesn't seem to absorb, reflect or emit light.
Dark matter and WIMPs
So what could dark matter be?
Some scientists think it could be made up of 'weakly interacting massive particles', or WIMPs.
WIMPs are heavier than protons, but don't interact much with other matter.
However, when two WIMPs collide, scientists predict the two particles will annihilate one another and release other particles, including gamma ray photons.
Researchers have been looking for years for these gamma rays in regions like the centre of our Galaxy, where dark matter is thought to be concentrated.
The smoking dark matter gun?
Professor Totani believes he may have finally detected the specific gamma rays predicted by the annihilation of dark matter particles, using data from NASA's Fermi Gamma-ray Space Telescope.
The Fermi Gamma-ray Space Telescope is a space observatory specifically designed to search for gamma rays in the Universe.
"We detected gamma rays with a photon energy of 20 gigaelectronvolts (or 20 billion electronvolts, an extremely large amount of energy) extending in a halolike structure toward the center of the Milky Way galaxy. The gamma-ray emission component closely matches the shape expected from the dark matter halo," says Totani.
He says the observed energy spectrum matches the emission predicted from the annihilation of WIMPs and that the estimated frequency of WIMP annihilation also falls within the range of theoretical predictions.
And, Totani says, these gamma-ray measurements can't be explained easily by any other known phenomenon.
"If this is correct, to the extent of my knowledge, it would mark the first time humanity has ‘seen’ dark matter. And it turns out that dark matter is a new particle not included in the current standard model of particle physics.
"This signifies a major development in astronomy and physics," said Totani.
More to do
Despite the results, these measurements have not yet been verified through independent analysis by other scientists.
Key will be determining that the radiation detected is a result of dark matter, rather than some other cosmic phenomena.
Totani says additional proof of WIMP collisions in other locations with a high concentration of dark matter would bolster his results.
For example, detecting the same energy gamma-ray emissions from dwarf galaxies within the Milky Way halo.
"This may be achieved once more data is accumulated, and if so, it would provide even stronger evidence that the gamma rays originate from dark matter," he says.
